One of the most common tasks is to generate the flat text of the email message
represented by a message object structure. You will need to do this if you want
to send your message via the smtplib module or the nntplib module,
or print the message on the console. Taking a message object structure and
producing a flat text document is the job of the Generator class.

Again, as with the email.parser module, you aren’t limited to the
functionality of the bundled generator; you could write one from scratch
yourself. However the bundled generator knows how to generate most email in a
standards-compliant way, should handle MIME and non-MIME email messages just
fine, and is designed so that the transformation from flat text, to a message
structure via the Parser class, and back to flat text,
is idempotent (the input is identical to the output) [1]. On the other hand,
using the Generator on a Message constructed by program
may result in changes to the Message object as defaults
are filled in.

bytes output can be generated using the BytesGenerator class.
If the message object structure contains non-ASCII bytes, this generator’s
flatten() method will emit the original bytes. Parsing a
binary message and then flattening it with BytesGenerator should be
idempotent for standards compliant messages.

Optional mangle_from_ is a flag that, when True, puts a > character in
front of any line in the body that starts exactly as From, i.e. From
followed by a space at the beginning of the line. This is the only guaranteed
portable way to avoid having such lines be mistaken for a Unix mailbox format
envelope header separator (see WHY THE CONTENT-LENGTH FORMAT IS BAD for details). mangle_from_
defaults to True, but you might want to set this to False if you are not
writing Unix mailbox format files.

Optional maxheaderlen specifies the longest length for a non-continued header.
When a header line is longer than maxheaderlen (in characters, with tabs
expanded to 8 spaces), the header will be split as defined in the
Header class. Set to zero to disable header wrapping.
The default is 78, as recommended (but not required) by RFC 2822.

The policy keyword specifies a policy object that controls a
number of aspects of the generator’s operation. If no policy is specified,
then the policy attached to the message object passed to flatten
is used.

Print the textual representation of the message object structure rooted at
msg to the output file specified when the Generator instance
was created. Subparts are visited depth-first and the resulting text will
be properly MIME encoded.

Optional unixfrom is a flag that forces the printing of the envelope
header delimiter before the first RFC 2822 header of the root message
object. If the root object has no envelope header, a standard one is
crafted. By default, this is set to False to inhibit the printing of
the envelope delimiter.

Note that for subparts, no envelope header is ever printed.

Optional linesep specifies the line separator character used to
terminate lines in the output. If specified it overrides the value
specified by the msg‘s or Generator‘s policy.

Because strings cannot represent non-ASCII bytes, if the policy that
applies when flatten is run has cte_type
set to 8bit, Generator will operate as if it were set to
7bit. This means that messages parsed with a Bytes parser that have
a Content-Transfer-Encoding of 8bit will be converted
to a use a 7bit Content-Transfer-Encoding. Non-ASCII bytes in the
headers will be RFC 2047 encoded with a charset of unknown-8bit.

Changed in version 3.2: Added support for re-encoding 8bit message bodies, and the
linesep argument.

Write the string s to the underlying file object, i.e. outfp passed to
Generator‘s constructor. This provides just enough file-like API
for Generator instances to be used in the print() function.

As a convenience, see the Message methods
as_string() and str(aMessage), a.k.a.
__str__(), which simplify the generation of a
formatted string representation of a message object. For more detail, see
email.message.

Optional mangle_from_ is a flag that, when True, puts a >
character in front of any line in the body that starts exactly as From,
i.e. From followed by a space at the beginning of the line. This is the
only guaranteed portable way to avoid having such lines be mistaken for a
Unix mailbox format envelope header separator (see WHY THE CONTENT-LENGTH
FORMAT IS BAD for details).
mangle_from_ defaults to True, but you might want to set this to
False if you are not writing Unix mailbox format files.

Optional maxheaderlen specifies the longest length for a non-continued
header. When a header line is longer than maxheaderlen (in characters,
with tabs expanded to 8 spaces), the header will be split as defined in the
Header class. Set to zero to disable header
wrapping. The default is 78, as recommended (but not required) by
RFC 2822.

The policy keyword specifies a policy object that controls a
number of aspects of the generator’s operation. If no policy is specified,
then the policy attached to the message object passed to flatten
is used.

Print the textual representation of the message object structure rooted
at msg to the output file specified when the BytesGenerator
instance was created. Subparts are visited depth-first and the resulting
text will be properly MIME encoded. If the policy option
cte_type is 8bit (the default),
then any bytes with the high bit set in the original parsed message that
have not been modified will be copied faithfully to the output. If
cte_type is 7bit, the bytes will be converted as needed
using an ASCII-compatible Content-Transfer-Encoding. In particular,
RFC-invalid non-ASCII bytes in headers will be encoded using the MIME
unknown-8bit character set, thus rendering them RFC-compliant.

Messages parsed with a Bytes parser that have a
Content-Transfer-Encoding of 8bit will be reconstructed
as 8bit if they have not been modified.

Optional unixfrom is a flag that forces the printing of the envelope
header delimiter before the first RFC 2822 header of the root message
object. If the root object has no envelope header, a standard one is
crafted. By default, this is set to False to inhibit the printing of
the envelope delimiter.

Note that for subparts, no envelope header is ever printed.

Optional linesep specifies the line separator character used to
terminate lines in the output. If specified it overrides the value
specified by the Generatoror msg‘s policy.

Write the string s to the underlying file object. s is encoded using
the ASCII codec and written to the write method of the outfpoutfp passed to the BytesGenerator‘s constructor. This
provides just enough file-like API for BytesGenerator instances
to be used in the print() function.

New in version 3.2.

The email.generator module also provides a derived class, called
DecodedGenerator which is like the Generator base class,
except that non-text parts are substituted with a format string
representing the part.

This class, derived from Generator walks through all the subparts of a
message. If the subpart is of main type text, then it prints the
decoded payload of the subpart. Optional _mangle_from_ and maxheaderlen are
as with the Generator base class.

If the subpart is not of main type text, optional fmt is a format
string that is used instead of the message payload. fmt is expanded with the
following keywords, %(keyword)s format:

This statement assumes that you use the appropriate setting for the
unixfrom argument, and that you set maxheaderlen=0 (which will
preserve whatever the input line lengths were). It is also not strictly
true, since in many cases runs of whitespace in headers are collapsed
into single blanks. The latter is a bug that will eventually be fixed.